An expanding sector of the electronic industry is involved with the design and production of equipment for systems deemed to have a "high availability application". The designers of such equipment strive to assure that the systems will not fail. High availability systems generally are called for by industries involved in telecommunications, banking, security, the internet and the like.
Inasmuch as the cognizant industry is aware that essentially all electrical equipment will fail at some point in time, design efforts have considered, for example, redundancy, now termed "N+1 redundancy". Additionally, over the recent past, failure warning systems and improved access for fault correction has been incorporated into the systems. Typically, these systems comprise a series of somewhat elaborate circuits which are mounted upon circuit boards the combination of which are referred to as "cards". These cards are mounted within the sub-rack or "card cage" of a housing referred to as a "chassis" or "sub-rack" within which additionally are mounted a backplane, serving to interconnect the cards and a power supply. Such power supplies have colloquially been referred to as "shoebox" power supplies since their size and shape has resembled such a container. In light of their bulk, the power supplies traditionally have been bolted to an associated chassis rearwardly of the backplane and coupled to it with cabling.
Over a period of time, a number of industry standards have been promulgated or are in the process of being developed both with respect to backplanes and their associated power supplies. Among these standards are "VME BUS", the term VME representing a backplane/bus structure referred to as "versa module Europe", developed by Motorola/Mostek/Signetics-Phillips A "VXI consortium" of industry and government evolved "VXI" specification for a VXI/VME back-plane/bus. More recently, a "Compact-PCI" standard has been promulgated by the industrial computer IC Manufacturers Group. In general, these specifications look to various aspects of system environmental requirements such as airflow, power source derived voltage outputs, connector pin assignments, protocols and the like. Specified power supplies are required with the standards which are manifested as a sequence of voltage outputs which must remain within defined tolerances.
As the systems have increased in complexity, a need was perceived for a chassis or mainframe structure having a controlled and monitored environment, both with respect to cooling airflow and with respect to power supply performance. Tracewell, in U.S. Pat. No. 5,168,171, issued in 1992, described a circuit enclosure which incorporates a microprocessor-driven support system functioning to monitor the status of power supplies, which provides an improved cooling airflow path and further provides temperature monitoring. Importantly, the status information and system controls were established at a user accessible supervisory panel mounted at the front face of the enclosure. Sold under the trade designation "Intelligent Mainframe", the improved enclosure design found ready acceptance in the electronics industry.
Providing such forward access has become an important aspect for high availability systems. Where a component such as a card or power supply fails or deteriorates to evoke a warning status, it is important that service personnel be capable of replacing it promptly. Being able to carry out that prompt servicing calls for front access to the components mounted in the mainframe. To further facilitate such rapid and relativity straight forward component exchange, standards organizations are commending to specify a feature wherein components are plugged into the system from the forward location while it is actively running. This feature is referred to as "hot swapping". One approach to providing power supplies which are accessible from the front of the mainframe is described in U.S. Pat. No. 5,940,288, issued Aug. 17, 1999 by Kociecki, entitled "Card Cage Mounted Power Supply With Heat Dissipating Architecture". The power supply described therein is thin, having a standard slot height and is inserted within a card slot in the same manner as a card. Heat removing airflow is provided to the power supply by the cooling fans which also are utilized to remove heat from the array of cards. Because the power supplies are connected into the backplane by a conventional pin array the necessity for bolted cable connections utilizing rather robust studs is eliminated and the capability for carrying out hot swapping is achieved.
As the systems at hand have become more complex, however, a concomitant requirement for additional card slot space has arisen. Thus, a need is present for a modular form of power supply which is forwardly accessible and exhibits a small size or form factor permitting it to be mounted adjacent the card cage without interfering with that function. This calls for not only diminutive volumetric sizes of the power supply but correspondingly small and preferably multitask handling structures.
Power supplies which are intended for use in more sophisticated electronic systems as referred to above, are required to be of a multi-output variety, for instance making available 5 volt, 3.3 volt and .+-.12 volt outputs to the system designer. Traditionally, those multiple outputs have been derived with circuits employing a singular step-down transformer combined with a single a.c.-to-d.c. conversion network. While this design approach conserves circuit space and cost, the multiple outputs which result are somewhat interdependent and only one of these outputs will be regulated. Such interdependence of the outputs often results in their degradation. Circuit alterations provided to attempt correction of these defects generally call for undesirable compromises. More operationally satisfying, mutually independent multiple output stages have called for separate, independent transforming and conversion networks with attendant large and now more unacceptable size and cost requirements.
Over the recent past, multiple output power supplies with mutually independent outputs have been introduced which achieve a somewhat reduced package size through the utilization of pre-manufactured switching converters, permitting improved power packaging densities. Manufacturers offer them as "bricks", the converters generally being rectangularly shaped packages of typically flat configuration (i.e., one-half inch thickness), one side of which incorporates a heat transfer surface generally formed of aluminum. With the introduction of the switching converters, improved packaging and cooling techniques have been developed. See in this regard: Tracewell, et al, application for U.S. Pat. No. 5,945,746, entitled "Power Supply and Power Supply/Backplane Assembly and System", issued Aug. 31, 1999; Tracewell, et al, U.S. Pat. No. 6,046,921 entitled "Modular Power Supply", filed issued Apr. 4, 2000; and U.S. Pat. No. 5,940,288 (supra).
Notwithstanding the provision of mutually independent multiple outputs, the size and costs of the devices remains the subject of the investigation. Smaller power supply sizes continue to be called for by systems designers. Such smallness in packaging with independent outputs combined with concomitant and important management of the heat generated by the supplies has remained a goal of power supply designers.
Another aspect of multiple output power supplies is concerned with their initial use during the development of an electronics system. System designers are called upon to compute the power demands associated with the system loads. The power supply manufacturers will provide the systems designer with an overall power supply rating in watts. However, since substantial variations occur with respect to the use of individual outputs, the power supplies are designed with power ratings for the individual outputs which, if combined as a whole, would exceed the overall power rating of the power supply. Thus, it is not unusual for the power supplies to be subjected to excessive power demands during such incipient design activities with resultant overheating and damage.